Divide surfaces of one space into separate surfaces to assign different boundary conditions or construction

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Hello Users, I use Sketch Up with OpenStudio plugin. I need to model a building with 64 apartments. The building has a basement too. But the basement isn’t completely under the ground. The basement is one thermal zone and one space. But the outside boundary conditions of the surfaces are different for the part under ground (Outside boundary condition: Ground) and the part above the ground (Outside boundary condition: Outside). Does anyone know, how can I separate the surrounding surfaces of my basement into two parts? Regards Arne